1. Field of Invention
This invention relates to the processing of standard silicon integrated-circuit (IC) chips or wafers containing precursors for micro-electromechanical systems (MEMS) structures co-integrated with standard CMOS-IC control or signal processing electronics, specifically, to etching silicon chips or wafers to convert MEMS precursors into functional MEMS structures while preserving aluminum bonding pads located on the chips or wafers.
2. Description of Prior Art
CMOS-IC Foundry MEMS
Parameswaran et al., Micromachined Thermal Radiation Emitter from a Commercial CMOS Process, IEEE Electron Device Letters 12, 57-59 (1991), describe a method for co-integrating MEMS structures with CMOS IC's on chips fabricated in a standard CMOS process by a commercial foundry service. Both the circuits and MEMS precursors were designed with the same standard IC CAD software. A special layer called OPEN was used to define the MEMS precursors. Following completely standard fabrication at the commercial foundry, the chips were etched in a mixture of ethylenediamine, pyrocatechol, and water (EDP) to remove the silicon below the MEMS precursor. What remained was a functional MEMS structure.
The EDP etchant was heated to over 90° C. while being stirred in a retort-type etching vessel fitted with a reflux condenser. Once the etching temperature had been reached, the chips to be etched were inserted into the retort for a predetermined etching period, typically less than one hour, after which they were removed. No photolithographic or masking steps were required before the chips were put into the retort and the etch was self-limiting. Therefore, precise control of the duration of the etch was not required. When the time required to convert the MEMS precursors into functional MEMS structures was less than about one hour, then the aluminum bonding pads that were connected to the co-integrated IC's usually were not damaged too much to prevent wirebonding.
The method of Parameswaran et al. is, however, not without problems. First, EDP is a toxic material that is difficult to handle. Some of its constituents have a high vapor pressure. It also forms a mist whenever the retort is opened at temperature, and whenever EDP is poured from one container to another. Misting appears to pose the most acute inhalation hazard. Second, while EDP etches silicon much more rapidly than aluminum, it still etches aluminum at a rate great enough to completely remove the bonding pads after not much longer than one hour, and partial etching of the bonding pads during the shorter time period may reduce the reliability of the bonding pads during the life of the packaged chip. Third, EDP is very dirty. It leaves a residue on the chips that cannot be removed with a simple rinse. If a chip is rinsed a few times in deionized (DI) water and then set in a beaker of fresh DI water for a number of hours, the water will become very dark indicating removal of residue that remained after the initial rinses. If all of this residue is not adequately removed by flowing fresh DI water over the chip for many hours, the chip will deteriorate over a period of months. Finally, etching at a temperature over 90° C. is inconvenient due to its proximity to the boiling point of water.
TMAH Etchants of Schnakenberg et al.
Schnakenberg et al., TMAH etchants for silicon micromachining, Digest of Technical Papers, '91, IEEE Int. Conf. on Solid-State Sensors and Actuators, 815-818 (1991), reported on the etching properties of different concentrations of tetramethylammonium hydroxide (TMAH) and dissolved silicon (silicate) in water. They reported that                they prepared their etchants by diluting commercial solutions of 25% TMAH (by weight in water) and carried out all etches at 80° C. with a reflux condenser without stirring,        the etchant pH, which was apparently measured during etching, decreased during etching and was the most important factor affecting the etching characteristics; the etch rate depended upon pH rather than the concentrations of TMAH and silicon dissolved in the etchant,        pyramidal hillocks bounded by (111) crystal planes formed on the etched surfaces when the etchant pH was less than 13, which corresponds to a TMAH concentration of about 20% in the absence of dissolved silicon; the density and size of the hillocks increased with decreasing pH,        the silicon (100) etch rate was about 39 μm per hour at a pH of about 11.8, which corresponds to an undoped TMAH concentration of about 2%, and the ratio of the silicon (111) to (100) etch rate was about 0.02 at this pH,        the silicon (100) etch rate was about 26 μm per hour at a pH of about 13, which corresponds to an undoped TMAH concentration of about 20%, and the ratio of the silicon (111) to (100) etch rate was about 0.04 at this pH,        the silicon (100) etch rate decreased monotonically as the pH was increased from 11.8 to 13.3,        the silicon (100) direction etch rate of TMAH is greater than that of EDP,        the etch rate for aluminum, which was mainly influenced by the concentration of dissolved silicon in the etchant and which was controlled by doping the etchant with silicon, could be reduced to a negligible value by dissolving silicon (as silicate) in a concentration that depended upon the TMAH concentration (over 90 grams per liter at 20% TMAH by weight,        the etch rate for the other standard layers used in CMOS integrated circuits was very low compared to the silicon (100) etch rate in the absence of dissolved silicon, and even lower in the presence of dissolved silicon,        
While providing some important advantages over the use of EDP, the procedure of Schnakenberg et al. also has some disadvantages. First, pH monitoring at high pH and high temperature is expensive and requires considerable care and frequent recalibration to be reliable. Secondly, hillocks formation is a serious problem because the etching process becomes unreliable. In fact, etching often ceases in the presence of hillocks, so the procedure of Schnakenberg et al. is really useful only when the pH is maintained above 13. TMAH is an expensive reagent, and using it above pH 13 requires a high concentration, which increases the etching cost significantly. Also, the selectivity against etching in the (111) direction is less than desirable at pH 13. Finally, Schnakenberg et al. didn't describe how they doped the etchant with sufficient silicon.
TMAH Etchants of Tabata et al.
Tabata et al., Anisotropic etching of silicon in TMAH solutions, Sensors and Actuators A34, 51-57 (1992), carried out a similar study to that of Schnakenberg et al. at four different etching temperatures, including 80° C., and came to similar though not identical conclusions. Besides reporting that they used silicon wafers, which they dissolved in the TMAH etchant, as a source of dissolved silicate, the most noteworthy differences that they reported included                their solutions had TMAH concentrations ranging from 5% to 40% that were obtained by dilution or condensation of commercial solutions of 25% TMAH (by weight in water); unlike Schnakenberg et al., they apparently did not measure the solution pH,        in the presence of no dissolved silicon (as silicate), the silicon (100) etch rate was about 90 μm per hour at a TMAH concentration of 5%, which is considerably greater than that reported by Schnakenberg et al.,        in the presence of no dissolved silicon, the silicon (100) etch rate was about 60 μm per hour at a TMAH concentration of 22%, which is also considerably greater than that reported by Schnakenberg et al.,        when the etched surfaces were covered with hillocks, the etch rate became very low,        the etch rate at a TMAH concentration of 22% at 80° C. is less than that of EDP, which is the opposite of what Schnakenberg et al. reported,        the aluminum etch rate showed a threshold-like behavior as a function of dissolved silicon; at low concentrations of dissolved silicon, the aluminum etch rate was comparable to that of a (100) silicon plane, and decreased by only a factor of 3 with the addition of 50 grams per liter of dissolved silicon, but decreased by a factor of 10,000 with the addition of 30 more grams of dissolved silicon.        TMAH is non-toxic. (THIS IS DEFINITELY FALSE, but because TMAH does not mist, and because its original components are much less volatile than those in EDP, it is much easier to handle safely than EDP. Its thermal decomposition products at 130° C., trimethylamine and methanol, which may be generated to some extent during etching, are also less hazardous, relatively speaking.)        it appears that Tabata et al. recommend use of a 22% solution of TMAH with at least 67 grams per liter of dissolved silicon operated at 90° C. for use as a silicon (100) etchant that will preserve the other layers present in standard CMOS IC fabrication processes.        
Actually, the procedure of Tabata et al. is so similar to that of Schnakenberg et al. that it has most of the same disadvantages, but it does appear to eliminate the disadvantage of pH monitoring. Another disadvantage of the procedure of Tabata et al. is that it requires dissolving a silicon wafer in the TMAH solution to prepare the etchant. (Schnakenberg et al. don't report how they obtain dissolved silicon.) Dissolving a wafer is inconvenient because it takes longer than to carry out a typical etch. It is also less convenient to handle silicon wafers than to handle reagents that are available from a liquid source such as the water solution of TMAH that these authors used. Yet another disadvantage of the procedure that is apparently recommended by Tabata et al. is that the etch must be carried out at 90° C., which is less convenient than a lower temperature because it is so close to the boiling point of water.
TMAH Etchants of Klassen et al., Tea et al., and Paranjape et al.
Klassen et al., Micromachined Thermally Isolated Circuits, Solid-State Sensor and Actuator Workshop, 127-131 (Hilton Head, S.C., 1996), reported successfully etching chips containing co-integrated MEMS precursors and CMOS circuits with a water solution of 5% TMAH by weight, 16 grams per liter dissolved silicon, and 5 grams per liter of ammonium peroxydisulfate (APODS) for 150 minutes at 80°. They also stated that that 40 grams per liter of dissolved silicic acid (SA) can be used instead of 16 grams per liter of dissolved silicon (obtained from dissolving a silicon wafer), that between 5 and 10 grams per liter of APODS was used, and that this concentration of APODS was sufficient to prevent hillock formation during the entire etching period.
The procedure of Klassen et al. improves upon that of Tabata et al. in that it uses considerably less expensive TMAH reagent. On the other hand, dissolving SA instead of dissolving silicon wafers introduces as many problems as it solves. SA is sold as a low density powder of very fine particles of SiO2.xH2O, where x is not precisely known. Therefore, SA is not really a reagent and cannot be used as source of a precisely known quantity of SIO2 in the TMAH etchant.
Furthermore, due to the small size of the SA particles, fluffy clouds of SA have a tendency to float into the air when it is poured or scooped. Besides being an inhalation hazard, this makes it very difficult to control the quantity of SA added to the etchant, which adds to the uncertainty in the quantity of silicate in the etchant.
Tea et al., Hybrid Postprocessing etching for CMOS-compatible MEMS, J. of MEMS, 6, 363-372 (1997), describe a modification of the procedure of Klassen et al. The major difference is that that Tea et al. used a higher pH solution, and added 16 g of 25% TMAH and 5 g of APODS about every 35 minutes during etches that lasted longer than 35 minutes after removing the sample holder. They did not replace the sample holder until all of the APODS was dissolved about 10 minutes after it was added to the etchant.
The additional APODS was needed because the lifetime of the APODS apparently decreases with increasing pH. The additional TMAH was used to compensate for the reduction in etchant pH caused by the ammonium ion contained in the APODS. According to Tea et al., their procedure is still far from ideal, suffering the same problems with SA as the procedure of Klassen et al.
Paranjape et al., Dual-doped TMAH silicon etchant for MEMS structures and systems applications, J. Vac. Sci. Technolo. A18, 738-742 (2000), describe comparisons of etching with high concentration of TMAH without silicate and APODS and etching with lower concentrations of TMAH, dissolved silicate, and APODS. Like Tea they add APODS at intervals during a long etch, but they do not describe their procedure.
Paranjape et al. recommend a 5% by weight solution of TMAH with 42.5 grams per liter of SA. Their procedure is very similar to that of Klassen et al. and Tea et al., and suffers from many of the same problems. Indeed, Paranjape et al. specifically state that the correct concentration of SA needed to preserve exposed aluminum layers varied from under 20 grams per liter to over 40 grams per liter depending upon the water content of the SA. In fact, they suggested that the proper amount of SA should be determined for each experimental case, which is very inconvenient.
Silicate Chemistry
G. Lagerstroem and N. Ingri, Equilibrium studies of polyions, IV. Silicate ions in NaCl medium, Acta Chem. Scand. 13, 758-775 (1959) and N. Ingri and G. Lagerstroem, Equilibrium studies of polyions, III. Silicate ions in NaClO4 medium, Acta Chem. Scand. 13, 722-736 (1959) point out that when silicon is dissolved in a strong base like TMAH, the resulting solutions consist of a number of different silicate species including Si(OH)4, SiO(OH)3−, SiO2(OH)22−, and Si4O6(OH)63−.
As long as the OH− concentration remains above about 2×10−3, all species present are in solution and the kinetics are fast. If the OH− concentration falls below this value, even momentarily, a precipitate of Si(OH)4 and silicate polymers will begin to form. The solution may remain clear for some time while the precipitate is forming, but will eventually become cloudy if the OH− concentration remains below about 2×10−3. If the OH− concentration is again raised above 2×10−3, the precipitate will redissolve, but only very slowly.
The chemical reactions among different silicate species are such that it takes more than one mole of a singly ionized base like TMAH to neutralize one mole of SA, and even more base is needed to bring the OH− concentration high enough to prevent precipitation of Si(OH)4 and silicate polymers. Thus, the pH of the solution is lowered from the pH before dissolution of the SA by an amount that depends upon the concentration of each silicate species.
The net effect is that there is a small window in which the solution pH is high enough to prevent precipitation of silicate and low enough to prevent attack on exposed aluminum. This means that the procedures of Klassen et al., Tea et al., and Paranjape et al., utilizing SA as they do, will suffer from reproducibility problems because the water content of the SA cannot be accurately determined prior to etching. Not only can such problems waste an expensive reagent, but they can also ruin fully fabricated wafers, which are very expensive.
To summarize, all of these authors report contradictory results and draw different conclusions, which suggests that the preparation and use of TMAH etchants is not simple, but requires great attention to detail. Schnakenberg et al. and Tabata et al. recommend using TMAH concentrations above 20%, whereas Klassen et al., Tea et al., and Paranjape et al. recommend TMAH concentrations below 10% with APODS to prevent hillock formation. All of the authors cited above recommend using dissolved silicate to preserve exposed aluminum, but Schnakenberg et al. do not describe their source of dissolved silicate. Tabata et al. report using silicon wafers. Not only are these much less convenient than a liquid reagent, they also take a very long time to dissolve. Klassen et al., Tea et al., and Paranjape et al. report using SA as a source of dissolved silicate. Not only is SA much less convenient than a liquid reagent, but it does not deliver a reproducible quantity of dissolved silicate to the etchant. Finally, neither Klassen et al., nor Tea et al., nor Paranjape et al. describe how they add APODS, which is sold as a powder, to the etchant. Klassen et al. add APODS only once, while Tea et al. and Paranjape et al. add APODS periodically during the etch, but only Tea et al. add TMAH when they add APODS to compensate for the pH reduction caused by the APODS.